Website Notes: Atomic Physics. 1. So far we are familiar with three atomic Models. The first was that of Democritus (460370) BC, the ancient Greek philosopher, who proposed that the smallest unit of matter was a small, single, indivisible particle, "atomos." 2. This belief lasted for almost 2300 years, until the 1890's when the British Physicist, J. J. Thomson discovered the electron using his CRT. His model was known as the "Plum Pudding" Model. 3. In the early 1900's, Ernest Rutherford, also from England proposed his Planetary Model, based on his "Gold Foil" Experiment. This atomic model was characterized as mostly empty space, but with a very dense, positively charged nucleus, and orbiting electrons. 4. While the Rutherford (Planetary) model focused on describing the nucleus, the electron was depicted as an orbiting planet. The flaw with the planetlike model is that an electron particle moving in a circular path would be accelerating. 5. An accelerating electron creates a changing magnetic field. This changing magnetic field would carry energy away from the electron, eventually slowing it down and allowing it to be "captured" by the nucleus. 6. In 1913, the Danish physicist Niels Bohr (18851962) managed to explain the a new atomic model as an extension of Rutherford's description of the atom. 7. Bohr agreed that the negatively charged electrons revolve about the positively charged atomic nucleus because of the attractive electrostatic force according to Coulomb's law. 8. But the electron can be taken not only as a particle, but also as a de Broglie wave (wave of matter) which interferes with itself. 9. The orbit is only stable, if it meets the condition for a standing wave: The circumference must be an integer multiple of the wavelength. The consequence is that only special values of radius and energy are allowed. 10. According to classical electrodynamics, a charge, which is subject to centripetal acceleration on a circular orbit, should continuously radiate electromagnetic waves. 11. Thus, because of the loss of energy, the electron should spiral into the nucleus very soon. By contrast, an electron in Bohr's model emits no energy, as long as its energy has one of the abovementioned values. 12. However, an electron which is not in the lowest energy level (n = 1), can make a spontaneous change to a lower state and thereby emit the energy difference in the form of a photon (particle of light). 13. By calculating the wavelengths of the corresponding electromagnetic waves, one will get the same results as by measuring the lines of the hydrogen spectrum. 14. We must not take the idea of electrons, orbiting around the atomic nucleus, for reality. Bohr's model of the hydrogen atom was only an intermediate step on the way to a precise theory of the atomic structure, which was made possible by quantum mechanics and quantum electrodynamics. 15. Still, the most important properties of atomic and molecular structure may be exemplified using a simplified picture of an atom that is called the Planetary Quantum or Bohr Model. 16. Again, this model was proposed by Niels Bohr in 1915 and although it is not completely correct, but it has many features that are approximately correct and it is sufficient for much of our discussion. 17. The correct theory of the atom is called Quantum Mechanics; the Bohr Model is an approximation to quantum mechanics that has the virtue of being much simpler to understand. 18. In the Bohr Model the neutrons and protons occupy a dense central region called the nucleus, and the electrons orbit the nucleus much like planets orbiting the Sun (but the orbits are not confined to a plane as is approximately true in the Solar System). 19. This similarity between a Planetary Model and the Bohr Model of the atom ultimately arises because the attractive gravitational force in a solar system and the attractive Coulomb (electrical) force between the positively charged nucleus and the negatively charged electrons in an atom are mathematically of the same form. 20. The form is the same, but the intrinsic strength of the Coulomb interaction is much larger than that of the gravitational interaction; in addition, there are positive and negative electrical charges so the Coulomb interaction can be either attractive or repulsive, but gravitation is always attractive in our present Universe. 21. Based on the spectrum of atomic hydrogen and the fact that each gas has a unique emission and absorption spectrum, Niels Bohr proposed his Quantum Mechanical Atomic Model instead of Rutherford's Planetary Model. 22. He proposed that electrons can move from one energy level to another by absorbing or emitting photons. His equations were: (a) for orbital radii. r_{n} = 5.3x10^{11} m x n^{2} , and (b) for the ionization energy associated with each level,E_{n} = 13.6 eV x 1/n^{2} , with n = 1,2,3,... , the energy level number. The electronvolt (eV) is the energy unit for electrons, 1 eV = 1.6x10^{19} J . 23. Werner Heisenberg (19011976) determined that it is not possible to know the exact position and momentum of the electron, the Uncertainty Principle. 24. Arthur Holly Compton (18921962) bombarded a graphite block with Xrays demonstrating the momentum of photons (The Compton Effect ). The equation is mv = p = h/λ . 25. James Chadwick (18911974) an original member of Rutherford's research team proved the existence of neutrons in 1932. 26. Light Amplification by Stimulated Emission of Radiation (LASER), which was explained by Einstein in 1917, was invented in 1960. Laser light is very directional, powerful, monochromatic, and coherent, making it very useful. Website Notes: Nuclear Physics. 1. Henri Becquerel (18521908) accidentally found that all compounds containing uranium emitted rays that penetrate and fog photographic plates, after examining a mysterious rock. 2. Ernest Rutherford (18711937) identified alpha, beta, and gamma radiation and used alpha particles to bombard gold foil. He found that most of an atom is empty space but contains a massive positively charged nucleus. 3. The Curies, Pierre and Marie, were the first to discover other radioactive elements, for example, Polonium and Radium. 4. The nucleus can be characterized by a mass number, A, an atomic number, Z, and a neutron number, N, with A = Z + N. Atoms having the same number of protons but different amounts of neutrons are called isotopes. 5. The nucleus of an atom contains most of the mass, consists of protons and neutrons, with protons and neutrons termed as "nucleons." 6. We use the Atomic Mass Unit (amu), or u, for nucleon mass. To convert just use the fact that 1 u = 1.6605x10^{27} kg. This means that we now have the mass of a proton as, 1 p = 1.007825 u, and a neutron, 1 n = 1.008665 u. 7. The change, transmutation, in an atomic nucleus can be natural or artificial. Enrico Fermi (19011954) successfully produced artificially radioactive elements in the laboratory. 8. Radioactive decay produces three kinds of particles: alpha, α, helium nuclei; beta, β, highspeed electrons; and gamma, γ, ray photons. 9. Bombardment of nuclei by protons, neutrons, alpha particles, electrons, gamma rays, or other nuclei can produce a nuclear reaction. 10. Linear accelerators, synchrotrons, and supercolliders produce highenergy protons and electrons which can collide with each other or an atomic nucleus. 11. Particle detectors include photographic plates, the GeigerMuller tube, scintillation screens, and the cloud chamber. 12. Alpha can be stopped by thick paper, beta by thick aluminum foil, and a few centimeters of lead will stop gamma. 13. During positron decay a proton changes into a neutron with the emission of a positron and a neutrino. 14. When matter and antimatter combine, all matter is converted into energy, or lighter matterantimatter particle pairs. By pair production, energy is converted into a matterantimatter particle pair. 15. The weak interaction operates in beta decay while the strong force binds the nucleus together. During beta decay a neutron changes into a proton and the nucleus emits a beta particle and a massless antineutrino. 16. The binding energy is the energy equivalent of the mass defect. The assembled nucleus has less mass than its constituent parts due to massto energy conversion, Binding Energy = (Δm)c^{2} , with Δm as the mass defect. 17. Nuclear reactors use the energy released in fission as heat to boil water, which produces steam, that turns turbine blades to run a generator. 18. The binding energy of the nucleus is the difference in energy between its nucleons when bound and its nucleons when unbound. Energymass equivalence can be computed using 1 amu = 931 MeV. 19. The halflife, T_{½} , is the time required for half the original nuclei of a radioactive substance to undergo radioactive decay. We use the equation A = A_{0}∙2^{n} where n is the number of halflives, and A indicating amount. 20. The decay constant, lambda, λ, indicates the rate of radioactive decay. Halflife can also be calculated by T_{½} = .693/λ . 21. Nuclear reactions involve a change in the nucleus and can be given by equations. In equations for nuclear reactions, subscripts and superscripts must agree on both sides. 22. In a nuclear equation the sums of the subscripts (atomic number or particle charge) on both sides of the equation are equal and the sums of the superscripts (mass number) on both sides of the equation are equal. 23. In fission, heavier nuclei split to form lighter nuclei and energy is released. In fusion, lighter nuclei combine to form heavier nuclei with more binding energy. Website Notes: Particle Physics. 1. Chemistry can be understood in the physics of 3 particles (proton, neutron and electron), and the influence of the electromagnetic force. 2. Nuclear physics can be understood in the physics of 4 particles (proton, neutron, electron and electron neutrino), and the influence of the strong and weak nuclear forces together with the electromagnetic force. 3. The Standard Model Theory (SM) of particle physics provides a framework for explaining chemistry and nuclear physics (low energy processes). It additionally provides an explanation for subnuclear physics and some aspects of cosmology
in the earliest moments of the universe (high energy processes). 4. Physicists currently believe there are three types of basic building blocks of matter (quarks, leptons, bosons). Quarks and leptons make up matter, which
is held together by bosons. Each boson is associated with a force.
5. The photon, the unit of the electromagnetic force, holds the electron to the
nucleus in the atom. The way these particles combine dictates the structure of
matter. 6. The Standard Model is conceptually simple and contains a description of the elementary particles and forces. The SM particles are the 6 quarks, which include the up and down quarks that make up the neutron and proton. 7. The 6 leptons include the electron and its partner, the electron neutrino. The 4 bosons are particles that transmit forces and include the photon, which transmits the electromagnetic force. Click HERE for Force descriptions. 8. With the observation of the tau neutrino at Fermilab, all 12 fermions and all 4 gauge bosons have been observed. Seven of these 16 particles (charm, bottom, top, tau neutrino, W, Z, gluon) were predicted by the Standard Model before they were observed experimentally! 9. There is one additional particle predicted by the Standard Model called the Higgs, which has not yet been observed. It is needed in the model to give mass to the W and Z bosons, consistent with experimental observations. 10. While photons and gluons have no mass, the W and Z are quite heavy. The W weighs 80.3 GeV (80 times as much as the proton) and the Z weighs 91.2 GeV. 11. The Higgs is expected to be heavy as well. Direct searches for it at CERN dictate that it must be heavier than 110 GeV. And to get full credit for homework make sure you follow these steps: (i) read the problem and identify the given variables (ii) determine what you are asked to solve for (iii) find the correct formula to use (iv) use algebra to isolate the unknown (v) substitutein the given information and simplify. View the NEW PowerPoint™Slides. Problem Set #2 (Test Review KEY). For the Lab Abstract template. Click HERE.
 

